Empty 'data-attr' value represents HTML5 Boolean attribute, not just emp...

According to [w3c](
http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attribute) empty attribute represents `true`, not just empty string. And if attribute is omitted, it represents `false`. The latter is not a big deal as currently it returns `undefined` which becomes `false` afterwards explicit conversion to boolean. The former still fails in that sense and this pull request is supposed to fix that.

Current status:
```javascript
element = jQuery("<div data-somewhat></div>");
should_be_true = element.data("somewhat"); // ""
should_be_false = element.data("omitted"); // undefined

!!should_be_true; // false, incorrect; fixed in the pull request
!!should_be_false; // false, correct
```